This invention relates to a valve apparatus for a fluid. Various known valve apparatuses of such type have the drawbacks that where a valve is slightly opened, an unstable region appears in which a valve body shakes; and where operated under such unstable condition, the valve apparatus starts shaking, which gives rise to noises and damages not only the valve seat and valve body put also the whole valve apparatus. To avoid the above-mentioned drawbacks, it has been proposed to apply a tough material to those parts of a valve apparatus which are ready to be damaged. However, such a valve still has the disadvantage of rendering a valve apparatus expensive due to the high cost of such tough materal.
FIG. 1 shows the valve body and valve seat of the prior art valve apparatus by way of illustrating its drawbacks. The valve apparatus of FIG. 1 is the type used for control of, for example, a steam turbine apparatus of a power plant. Reference numeral 10 denotes a main valve seat; 12 a hollow main valve body movable in the axial direction C--C of the main valve seat 10; 14 an auxiliary valve seat formed in the main valve body 12; and 16 an auxiliary valve body movable through the main valve body 12 in the axial direction C--C of the main valve seat 10. The auxiliary valve body 16 is formed integrally with the lower end of a valve rod 18. Now let it be assumed that a region above the main valve seat 10 (indicated by P.sub.1) is supplied with a fluid (steam in this case) having pressure p1, and an opening P.sub.2 bored in the main valve seat 10 in the axial direction C--C thereof is filled with steam having pressure p2. Where, under this condition, the valve rod 16 is moved downward of FIG. 1, the main valve body 12 contacts the main valve seat 10, and the auxiliary valve body 16 abuts against the auxiliary valve seat 14, then the valve apparatus is closed.
Where the valve rod 18 is pulled up under the above-mentioned condition, then the auxiliary valve body 16 is removed from the auxiliary valve seat 14 to provide an auxiliary fluid pass between said auxiliary valve body 16 and auxiliary valve seat 14. As a result, steam runs through said pass in the direction of the indicated arrows. This flow is thereinafter referred to as "a side stream". This side stream runs downward of FIG. 1 through an opening P.sub.2 provided in the main valve seat 10. Where the valve rod 18 is further lifted under the above-mentioned condition, then, the main valve body 12 is pushed upward by the shoulder 16a of the auxiliary valve body 16 and leaves the main valve seat 10. As a result, a fluid pass (hereinafter referred to as "a main fluid pass" as against the auxiliary fluid pass) is produced between the main valve body 12 and main valve seat 10. Accordingly, a main stream is generated which runs obliquely downward of FIG. 1 through an opening P.sub.2 provided in the main valve seat 10.
With the prior art valve apparatus, the main valve body 12 and main valve seat 10 are separated from each other with a small clearance. Where, therefore, the ratio of p2/p1 has a relatively small value, then the main stream flows symmetrically with respect to the axial line C--C of the main valve seat 10, as indicated by broken lines. A main stream indicating this flow pattern is referred to as "a main stream of a first flow mode". A further rise of the valve rod 18 increases an opening between the main valve body 12 and main valve seat 10 (hereinafter referred to as "a valve opening") and consequently the value of the ratio of p2/p1. As a result, the main stream may runs asymmetrically with respect to the axial line C--C as indicated by solid lines in FIG. 1, that is, in a second flow mode. Where the above-mentioned valve opening reaches a certain extent, then the main stream fluctuates between the first and second flow modes. As a result, the main valve body 12 vibrates relative to the main valve seat 10, and similarly the auxiliary valve body 16 shakes relative to the auxiliary valve seat 14. The above-mentioned event results in the harmful shaking of the prior art valve apparatus. This shaking gives rise to objectionable noises, mechanically damages the valve seats and valve bodies, and leads to variation in the pressure on the fluid outlet side of the valve apparatus. The aforesaid auxiliary valve body 16 and auxiliary valve seat 14 jointly constutute the known pilot valve positioned above the main valve body 12 and main valve seat 10 to allow for the easy separation of the main valve body 12 from the main valve seat 10.